64 ELEMENTS OF WATER BACTERIOLOGY 



agar. He emphasizes the recommendation of Jackson 

 that the agar used should be dried at 105 C. for 

 30 minutes, as commercial agar itself contains more or 

 less water. 



The period of incubation ordinarily adopted for body- 

 temperature counts is 24 hours. Lederer and Bach- 

 mann (1911) find that with sewage effluents a 48-hour 

 period at 37 may yield counts from two to six 

 times as high as those obtained in 24 hours; it is ques- 

 tionable, however, whether the higher counts thus given 

 would compensate for the loss of time. The adoption 

 of a 24-hour period by the Standard Methods Com- 

 mittee in any case represents an almost universal 

 practice. 



In using agar plates at 37 difficulty is some- 

 times caused by the spreading of colonies of certain 

 organisms over the surface of the plate in the water 

 of condensation which gathers; this may be avoided 

 by inverting the plates after the agar is once well set, 

 or still better by the use of plates provided with earthen- 

 ware tops, as suggested by Hill. The porous earthen- 

 ware absorbs the water which condenses on it, the 

 surface of the plate remains comparatively dry, and the 

 percentage of " spread " plates is reduced from 30 

 per cent to i per cent (Hill, 1904). Special pains must 

 be taken, however, to keep the atmosphere in the 

 incubator nearly saturated with moisture or errors will 

 be introduced by the excessive evaporation of the 

 medium used. 



Use of Litmus Lactose Agar. Additional evidence 

 as to the character of a water sample may be 



